The present invention relates generally to controls for a refrigeration device, and more particularly to an adaptive defrost control for a refrigerator which varies the duration between defrosting cycles based upon the length of the time required to remove a frost load during the prior defrosting cycle.
Refrigeration controls are known wherein defrosting of an evaporator by defrosting apparatus is accomplished at periodic intervals. For example, Oishi et al U.S. Pat. No. 4,432,211 discloses a defrosting apparatus for removing frost deposited on a cooler of a refrigerator. The defrosting apparatus includes a defrosting heater which is periodically energized and is of the self-controlled type wherein the current through the heater is a function of the temperature thereof. The heater is de-energized at the end of a defrosting cycle by a control when the rate of change of current through the defrost heater exceeds a predetermined value.
A further type of defrost control is disclosed in Phillips U.S. Pat. No. 3,335,576. This defrost control utilizes a pair of thermistors which are disposed in heat transfer relationship with an evaporator coil of the refrigerator. When one of the thermistors senses a predetermined low temperature, a defrost cycle is initiated, which cycle is discontinued when the temperature sensed by the other thermistor reaches a predetermined high temperature.
A still further type of defrost control is disclosed in Allard et al U.S. Pat. No. 4,251,988. This defrost control is referred to as an "adaptive" defrost control since it establishes the time between succeeding defrosting cycles as a function of the length of time that the defrost heater was energized during the first defrosting cycle.
A more sophisticated type of adaptive defrost control is disclosed in Tershak et al U.S. Pat. No. 4,481,785, assigned to the assignee of the instant application. This adaptive defrost control varies the length of an interval between defrosting cycles in accordance with the number and duration of compartment door openings, the duration of a previous defrosting cycle as corrected by the temperature of the evaporator prior to defrost and the length of time the compressor has been energized.
Some of these prior types of controls rely upon the detection of the temperature of a component (such as an evaporator) by means of a thermistor. Various methods have been devised to determine the temperature to which a thermistor is exposed. For example, Baker U.S. Pat. No. 4,488,823, assigned to the assignee of the instant application, discloses a method wherein a capacitor is charged through a reference resistor and the length of time required to charge the capacitor to a predetermined level is measured to obtain a reference charge time. The capacitor is then discharged and subsequently charged through the thermistor whose temperature is to be determined. The length of time required to charge the capacitor to the predetermined level is again measured to obtain a temperature charge time. The temperature of the thermistor is determined by computing the difference between tween the reference charge time and the temperature charge time divided by the sum of the times.